1. Field of the Invention
This invention pertains to light dimming systems and more particularly to a system suitable for dimming one or more flourescent lamps, although also being suitable for dimming incandescent and high intensity, gaseous discharge lamps.
2. Description of the Prior Art
Light intensity of a lamp is dependent, after it reaches normal operation, on the power delivered to the lamp. That is, the greater the power, the brighter the lamp. It is possible to put a variable resistor in series with such a lamp for limiting the power to the lamp, and hence, varying the lamp intensity. However, such a device has several shortcomings. The primary shortcoming is that a resistor used in this fashion dissipates heat and, therefore, provides dimming at a loss in efficiency. Second, a variable resistor alone does not provide means for automatically adjusting light level to compensate for the brightness of an illuminated area from sunlight or other source external to the system being controlled.
Another procedure that has been employed in limiting power to the lamps circuit is to provide current to the ballast-and-lamp network only during a portion of each cycle of line current. This can be done by switching the current off each time there is a zero-crossing of line current and then switching the current on at a predetermined time after the zero-crossing time. The portion of on time determines the amount of average power applied each cycle and, hence, the brightness of the lamp or lamps. The problem with this approach is that too large a portion of non-applied current each cycle in the vicinity of the zero-crossing event prevents the lamp network from lighting the lamps at all.
It has also been observed that notching in the vicinity of a sine wave peak, especially when small notches are taken to produce small amounts of dimming, causes undesirable rapid jumps in power. The jumps are undesirable because they can cause damages surges to the lamp structure.
Therefore, it is a feature of the present invention to provide an improved system for dimming fluorescent or other lamps that provides improved and acceptable variable notching of the voltage applied to the lamp network for reducing the overall power applied thereto.
It is another feature of the present invention to provide an improved system for dimming fluorescent or other lamps by controlling the amount and location of a variable notch within a voltage cycle for the voltage applied to the lamp network, thereby limiting the overall power applied thereto without risking turning off the lamps or harmfully surging the lamps.
It is yet another feature of the present invention to provide an improved system for dimming fluorescent or other lamps by automatically controlling the amount of power applied to the lamp network from each cycle of line voltage as controlled by a photosensitive input means that senses the amount of ambient light.
It is still another feature of the present invention to provide an improved system for dimming fluorescent or other lamps only after stabilized operation has been attained.
It is yet another feature of the present invention to provide an improved system for automatically dimming fluorescent or other lamps controlled by an input that is the result of a comparison between a photosensed input and a standard input.